In order to do this, you need to turn on SSR for the particular joint. If this is not done, the strength of the joint is not reduced as part of the SSR analysis.
Applying SSR for joints in Define Joint Properties dialog
In addition, it has been found that when using joint elements (either for rock slopes or when using structural interfaces when modeling geotextiles), a lower tolerance should be used. Joint elements also tend to take more iterations to converge. A starting point could be a tolerance of 1e-4 and 1500 iterations, then 1e-5 for the tolerance with 5000 iterations. The tolerance and maximum number of iterations is specified in the Advanced SSR Settings dialog.
Change convergence parameters in the Advanced SSR Settings dialog